Since the dealer has to draw up to 17, he is more likely to bust if there is a higher percentage of face cards (10, J, Q, K, A) to low cards (2-6). Lets say your card count is 5 and you raise your bet proportional to your count.The dealer shows a 16 (assuming the hidden card is 10) and you have a 12. Since it is more likely that the next card will be 10 (hence your card count), you would stand because the dealer's percentage to bust is higher. Card counting basically uses probability as opposed to the traditional way of playing (assuming the hidden card is ten and hitting if you are below the dealers predicted total) to gain an advantage from a statistics standpoint.